Scanned light beam imaging is employed for various purposes including electrophotographic printing. In such systems used for printing, a light beam generated, for example, by a laser is selectively modulated to vary its amplitude while it is scanned laterally across a moving photoconductor to selectively discharge the photoconductor. Such systems have been binary in nature (“on” or “off”) to accomplish printing of character information by selectively either exposing the photoconductor or leaving it unexposed. To insure complete exposure, adjacent scan lines are usually overlapped slightly. Thus, any slight misalignment of the scans or change in beam size would not be noticed.
Therefore, a partial warranty requires stable and repeatable devices. Many well-known effects, however, adversely impact the stability and repeatability of Xerographic engines. For example, photoreceptor discharge curves are unstable due to charge trapping and poisoning. Develop-ability changes as toner concentration changes. It also is affected by the toner size distribution changes produced by the size selective nature of most Xerographic development systems; by toner and developer degradation due to the impaction and contamination they experience in the extremely harsh environment of the developer housing; and by roll-to-photoreceptor spacing variation due to mechanical problems like roll runout. The charging process is also affected by dirt on the coratron wires.
In addition, there are many other sources of instability and non-repeatability. These instabilities would not matter if the color shifts they produce were not visible. Unfortunately the human eye is exquisitely sensitive to color shifts. Trapping is a device specific command that compensates for registration when two colors are placed next to each other. Instead of allowing a white space to open up when the registration is imperfect, trapping swells each of the two color areas so that an overlap area occurs between butting color areas, which absorbs the misregistration. In anticipatory composition, the creator of the electronic document uses definitions and coding within the document based on the output media selected. This controls the document output, but also limits how the document can be printed or distributed.
It is therefore an object of the present invention to eliminate the perception of gaps in color printing in such optical imaging systems without requiring substantial tightening of allowable optical or electromechanical tolerances.